So the other day I had a pretty big crash into mud..
The solution is to strip the motors apart, get a really hot container of water (boiling) and then jiggle it in until you get all dirt out.
I had to use a toothbrush in some parts (be careful)
Once you have got rid of dirt, you can also give it a blow with compressed air.
You will need to lube up the bearings again, I used some WD 40 and cleaned the whole motor as well with it then dryed it.
After I had pulled most dirt off.
Any suggestions or questions let me know